It was originally a trilogy that wrapped up in the three initial novels, but she later wrote sequels. I own the lot of her books, but I agree that the sequels, while nice to read to find out what happened next to the characters, are just not as engaging and compelling as the original story.
If you're okay with an sf suggestion, I think that Robert J. Sawyer's Golden Fleece
comes pretty close. One of the viewpoint-narrative characters does have a vested interest in steering things in a particular direction, and I would definitely not call the ending a triumph for either "side". And it's a rather good locked-room murder mystery set on a spaceship, to boot, and the whydunnit will probably surprise you.
His The Terminal Experiment
also comes sort of close-ish.
To get an idea of how close-ish I mean, perhaps you might like to read his award-nominated novellas Identity Theft and The Hand You're Dealt, free online at his website
. Oh, and I think Just Like Old Times is very close to what you originally had in mind.